1. Field of the Invention
This invention relates to a method for improving the discharge of bulk cargos of minerals, agricultural and forestry products from railroad cars, trucks, conveyors and silos. More particularly, it relates to a method to facilitate the unloading of cohesive, sticky or frozen bulk cargos of commodities such as coal, corn, wood chips and the like, and to improvements in the unloading of bulk cargos from their shipping containers.
2. Prior Art
As an example of bulk handling and shipping of materials, it is now commonplace to ship by rail, cargos of minerals such as coal in lots of 10,000 tons using 100 to 120 railroad cars coupled together as a unit train that repeatedly travels between mine and destination at frequent intervals. For efficiency of material handling and to maximize the repeated and continuous use of transportation equipment, a unit train cargo of 10,000 tons of coal often unloads at destination while moving slowly over the cargo dump site. Systems have been perfected and installed for the rapid unloading of cargos in bulk; these, however, require that the cargo flow freely on command as each railroad car or truck passes over the system's cargo discharge site.
Two major systems are used for unloading bulk cargos from railroad cars. One system inverts the car (sometimes several cars simultaneously usually without uncoupling) by rotating it about 160.degree.; this system can handle nearly all types of open-top cars. The other system uses cars having bottom or hopper doors that are opened mechanically or automatically as the train passes over the dump site. Both systems of unloading frequently fail to entirely discharge the contents of each car due principally to the sticky characteristics of small particles contained in bulk cargos, friction and cohesion of these particles with container surfaces, and freezing of some cargos in severe winter weather.
Bulk cargos are frequently stored before and after transit in silos where they arch against the silo walls and fail to flow readily from chutes at the base of silos, unless mechanically assisted.
The adhesion of cargo particles to the wall surfaces and to the bottoms of their containers slows and frequently prevents their complete discharge during unloading. As a result each railroad car of a unit train of coal, for example, often does not discharge at destination its total cargo, but instead returns to the mine with significant amounts (up to 20 tons) of coal in freezing weather, some of which may become lost along the way due to jolting, air currents and the sluicing action of rain. This is a wasteful practice that can be harmful to the environment. Such residues of cargo clinging to the surfaces of containers, if not lost en route, become a contaminant in the bottom of a railroad car, truck or conveyor when next loaded.
Noisy electric and pneumatic vibrators are used frequently to shake railroad cars vigorously and vibrate chutes to dislodge cargos from their containers at dump sites, and to make bulk cargos dislodge from their containers, hoppers, chutes and silos. Air cannons are sometimes used for freeing material at large silos near their bottom ends where bulk material, such as coal, tends to adhere to the silo's walls to form an arch or blockage. Explosives are sometimes used to dislodge by vibration or to shoot a projectile into the hang-up of material above a chute or discharge point to bring down a blockage. This use of explosives can be hazardous.
Problems caused by bulk cargos that stick to the walls and bottoms of their containers and to the walls and chutes of silos, are compounded during severe winter weather when freezing conditions prevail. Moisture lodged between individual particles of cargo freezes, particularly where the particles contact the surfaces of their shipping containers, storage silos or chutes, to which they freeze. Under winter conditions it often becomes necessary to thaw each railroad car in a thawing shed before its contents of coal, for example, can be discharged at destination. Obviously this is a time consuming and costly procedure, consuming calorific energy.
Chemical means are sometimes used, especially with bulk cargos of coal, to prevent freezing during winter weather. In a sub-freezing environment, surface moisture freezes and bonds individual particles together and to the metallic surfaces of railroad cars and trucks and other container surfaces. Whenever the handling of bulk cargos containing surface moisture is confronted with sub-freezing conditions, it will freeze to its container walls, in railroad cars, trucks, silos, chutes, conveyor belt surfaces, etc. To retard this freezing, particularly with cargos of coal, freeze conditioning agents are used (usually glycol or oil). These are sprayed onto the stream of bulk material as it passes through a spray chamber or fog zone of freeze conditioning agent, usually while the bulk cargo is loaded into its container for transport in freezing weather. This method uses excessive amounts of the agent because the entire cargo is treated, including material that is loaded in the center of the container which is thus insulated from freezing and need not be freeze-proofed.
Oil base freeze-proofing agents lose their effectiveness when temperatures fall below 20.degree. F. and they are, therefore, generally not satisfactory.
Calcium chloride based products are also used to defreeze after bulk cargos have frozen to the sides of their containers. They are unsatisfactory because they are corrosive to the metallic walls of containers.
Since container surfaces tend to slow the flow of materials in bulk at their points of discharge, the bottoms of chutes, silos, trucks and railroad cars are sometimes steeply inclined to encourage material discharge. Steep angles of surfaces or metallic slides at drawpoints at the bottom of railroad cars, for example, facilitate the discharge of coal cargos from the bottom of the car when its dump doors are mechanically opened automatically for rapid unloading of cargo. The use of relatively steeply inclined surfaces or slope plates (inclined up to 60.degree.) at the floor of railroad cars to improve their unloading, however, sacrifices cargo capacity of the car. Part of the area of roof-like gable structures used as false bottoms to cover the floor of cars, silos and chutes at discharge points could advantageously be used for stowage of additional cargo. This loss of cargo carrying capacity can be significant in the case of railroad cars designed to carry bulk commodities such as coal or grain because the unit train of many cars makes numerous repetitive journeys from mine to destination, and any loss of carrying capacity through inherent design is cumulative. Additionally, the metal sheets of the false bottoms and slope sheets add burdensome weight and construction expense to railroad rolling stock.
Safety is also a factor in the need for unrestrictive flow of bulk materials from their containers. The arching or hangup of substances in bulk at the time of discharge from a drawpoint, for example, at a silo, creates a temporary void at its base which can collapse without warning with an outrush of explosive dust for some bulk materials such as coal or grain. Avoidance of this potential disaster is currently sought by using steeply inclined bottom surfaces in some instances, but the present state of the art becomes inadequate when materials naturally adhere and arch over to the sides of their containers or silos.